It's definitely more kaiseki than anything else and not at all like prix fixe. Most prix fixe that I've experienced is 3-5 courses with choices of several dishes within each course for a set price. That is definitely not what Urasawa is.

Below is wikipedia's definition of kaiseki which includes the order that they may be presented. You can correlate many of Hiro-san's courses to the standard kaiseki menu. He usually lacks a grilled fish course and a rice course but he starts with an appetizer, sushi he serves at the end, has a sashimi course, has a lidded course, a hot pot course, a steamed course, and even a pickled course which he put towards more towards the beginning.

===================

OrderOriginally, kaiseki comprised a bowl of miso soup and three side dishes.[7] It has since evolved to include an appetizer, sashimi, a simmered dish, a grilled dish, and a steamed course[7], in addition to other dishes at the discretion of the chef. [8]

Sakizuke (先付?): an appetizer similar to the French amuse-bouche. Hassun (八寸?): the second course, which sets the seasonal theme. Typically one kind of sushi and several smaller side dishes. Mukōzuke (向付?): a sliced dish of seasonal sashimi. Takiawase (煮合?): vegetables served with meat, fish or tofu; the ingredients are simmered separately. Futamono (蓋物?): a "lidded dish"; typically a soup. Yakimono (焼物?): Broiled seasonal fish. Su-zakana (酢肴?): a small dish used to clean the palate, such as vegetables in vinegar. Hiyashi-bachi (冷し鉢?): served only in summer; chilled, lightly-cooked vegetables. Naka-choko (中猪口?): another palate-cleanser; may be a light, acidic soup. Shiizakana (強肴?): a substantial dish, such as a hot pot. Gohan (御飯?): a rice dish made with seasonal ingredients. Kō no mono (香の物?): seasonal pickled vegetables. Tome-wan (止椀?): a miso-based or vegetable soup served with rice. Mizumono (水物?): a seasonal dessert; may be fruit, confection, ice cream, or cake.

But you still have to somehow account for the next 25 (or 30 or 35) sushi/sashimi/kobe beef/etc. courses that he serves following the kaiseki portion of the dinner. So that's where the "hybrid" part comes in (my terminology).

The sushi and sashimi are part of a standard Kaiseki. Kobe beef is part of the shabu shabu/hot pot course. I take back my previous comment about him not doing a grilled fish course. His hot stone grilled toro would be his grilled fish course. I was thinking more in the traditional sense as in grilled whole fish. That clever Hiro-san... I agree its not just kaiseki, I agree you could call it kaiseki + sushi omakase hybrid. But it's definitely not prix fixe as I understand prix fixe.

Chef's choice or chef's tasting or chef's pleasure would be more of an umbrella term. Kaiseki would be more specific because of the the progression of dishes and certain dishes required to be considered "kaiseki" (as is my understanding of the term). So you could say that all kaiseki menus are "au plaisir du chef" but not all menus au plaisir du chef are kaiseki.